Immunodiagnostic methods have proved to be of great use both within and outside of clinical areas. Food testing, environmental testing and forensic applications are but some of the applications. The robustness, precision and convenience of the methods have led to applications ranging from kits for home use to sophisticated laboratory auto-analysers. In particular, the development of immunometric technologies for large molecular weight analytes has been outstandingly successful. A particular advantage of such technologies is that they produce an increasing signal with increasing concentration of analyte.
Classical competitive assay formats are very widely employed but have a fundamental difficulty in that they rely on setting up competition for a binding site between an analyte and an analyte analogue and thereafter measuring how much analyte analogue has been bound to the binding site. Thus they do not directly measure how much analyte has been bound. Such a method is described in Chapter 1 of ELISA and Other Solid Phase Immunoassays, Ed DM Kemeny & S J Challacombe, pub. Wiley, 1988. Other indirect assays which measure the number of binding sites not occupied by analyte are described in WO 95/04931 and WO 92/19973.
U.S. Pat. No. 5,641,690 discloses a method for determining an analyte using a specific binding agent. The analyte is mixed with an analyte analogue and contacted with the binding agent. Complexes between analyte and binding agent are detected by an antibody, which is typically specific for a site on the binding agent. The analyte analogue, however, is designed such that it prevents binding between antibody and binding agent when it is bound to the binding agent. This assay provides a way for directly measuring the number of binding sites occupied by analyte, thus enabling more sensitive and precise assays than those which rely on indirect detection of analyte binding. However these assays rely on being able to generate an antibody which binds sufficiently close to the analyte binding site for its interaction with the binding partner to be blocked by the chosen analyte analogue. This can be both time-consuming and costly.